1 (1 pound) beef top sirloin, thinly sliced
2 small onions, sliced
2 green bell peppers, sliced
1 cup barbeque sauce (such as Bull’s-Eye(R) Texas-Style Bold Barbeque Sauce)
8 (10 inch) flour tortillas
2 cups shredded Cheddar cheese
Preheat oven to 425 degrees F (220 degrees C).
Heat a skillet over medium heat; cook and stir beef until browned, 5 to 7 minutes. Add onions and green bell peppers to beef; cook and stir until softened, 5 to 10 minutes. Pour barbeque sauce over beef mixture and simmer until sauce is slightly reduced, about 10 minutes.
Lay 4 tortillas on a baking sheet and top each with beef mixture and Cheddar cheese. Top each Cheddar cheese layer with a tortilla.
Bake in the preheated oven for 10 minutes; flip the quesadillas and cook until cheese is melted, about 5 minutes more.